The invention uses various materials which are electrically either conductive, insulating or semiconducting, although the completed semiconductor circuit device itself is usually referred to as a "semiconductor". One of the materials used is silicon, which is used as either single crystal silicon or as polycrystalline silicon material, referred to as polysilicon or "poly" in this disclosure.
In many transistor circuits, it is necessary to form both n-channel transistors and p-channel transistors, in a complimentary metal oxide semiconductor (CMOS) circuit.
In the fabrication of a CMOS circuit, an n-well is first formed by masking a semiconductor wafer and implanting an impurity. The impurity is intended to infiltrate to a sufficient depth to define the n-well, as an n- material of sufficient depth to permit the functioning of the p-channel transistor circuit.
A typical dopant used is phosphorus, which has a characteristic of being fairly easy to control in depth and distribution.
After forming the n-wells, a thin layer of oxide, in the range of 250 .ANG., is grown on the wafer to form an initial oxide layer. The nitride is deposited and the wafer is then masked again for the purpose of defining active areas (AA). The mask is stripped and a photomask is then applied over n-wells. A boron implant is applied to the p-type area, with the boron functioning as a channel stop implant. The boron does not penetrate the nitride and, therefore, the nitrided areas do not receive the channel stop implant. The photoresist is then stripped and fieldox (field oxide) is grown in areas which are not layered with nitride. The wafer is then masked again in order to shield the n-well and a punch-through implant is applied to the n-type transistors, using boron dopant.
As shown in FIG. 1, the relationship of the length of a transistor and threshold voltage is not significant until the transistor becomes fairly short. At that point, the threshold voltage rapidly decreases along with the rapid decrease in length. This can be seen by reading the graph from right to left, where one is observing the effects of the reduction in length. FIG. 2 shows the subthreshold characteristics of normal-channel and short-channel transistors. A normal channel tends to present a step in conductivity, as is the expected characteristic of the transistor. Short-channel transistors, on the other hand, behave more as a resistor than a transistor when compared to the behavior of a normal-channel transistor.
In sub-micron transistors, the punch-through implant is necessary in order to avoid short-channel effects in the n-channel transistors. In the short-channel effect, the space between transistor source and drain is short enough that the potential between source and drain may be sufficient to cause the transistor to conduct, due to so-called drain induced barrier lowering. Short-channel effects include threshold voltage roll-off and drain-source punch through at high drain voltage.
In order to increase the n-channel field threshold voltage, one would increase field oxide thickness and/or implant impurities below the field oxide. Boron is used as an impurity in a punch-through implant process, wherein sufficient energy is applied during implant to cause the boron to penetrate the field oxide. Therefore, the high energy boron punch-through implant can both reduce n-channel short-channel effects and at the same time, improve n-channel field transistor threshold voltage.
It is therefore an object of the present invention to reduce the short-channel effects of submicron transistors. It is also desirable that the process of manufacturing the transistors uses a minimum of mask steps; therefore, any modification of the process should ideally not increase mask steps.
Blanket implant some boron through the field area prior to growth of field oxide, in order to raise n-channel field transistor threshold voltage V.sub.T. P-channel field transistor V.sub.T may be degraded by this blanket boron implant, but will be improved by the use of arsenic in n-wells. N-channel field transistor V.sub.T is further increased by the use of boron high energy punch-through implant.
The choice of dopants is made in accordance with the electrical effects of the dopant and the ease at which the dopant is implanted. Since the ease of implant affects the distribution of the dopant, this, of course, also affects the electrical characteristics of the device. As mentioned, phosphorus is a fairly easy material to implant with in that it requires a relatively low energy to penetrate the silicon wafer to a desired depth. Other dopants, such as arsenic, require more energy and tend to concentrate at a certain level; this level is determined by the amount of energy used in implanting the dopant.
Since phosphorus is said to have a high diffusion coefficient, meaning that phosphorus is relatively easy to be diffused by implanting into the wafer. Arsenic, on the other hand, has a lower diffusion coefficient. Applying more energy to diffuse arsenic results in the arsenic concentrating at a different level, a phenomenon which is referred to (at least here in Idaho) as the snowplow effect.
In the prior art, n-channel transistors are provided with a channel stop implant by first photomasking over the p-channel areas, and then applying the implant. The implant is a boron implant which creates a p+ material under areas which will eventually be defined as field by growing fieldox. The mask is used to prevent the boron from penetrating the transistor areas and thereby shorting the p-channel transistors.
From economical points of view, it is desirable to omit the number of mask steps used for fabricating integrated circuits. Therefore, any solution to the short-channel effect should not result in substantially increasing masking; ideally it should result in a decrease in mask steps.